Radiation generated by the impact of a charged particle, such as an electron, on a target, such as tungsten, referred to as Bremsstrahlung radiation, is commonly used in the non-invasive inspection of contents of objects, such as luggage, bags, briefcases, cargo containers, and the like, to identify hidden contraband at airports, seaports, and public buildings, for example. The contraband may include hidden guns, knives, explosive devices, illegal drugs, hazardous chemicals, hazardous biological agents, and weapons of mass destruction, such as a nuclear or a “dirty” radioactive bomb, for example. One common inspection system is a line scanner, where the object to be inspected is passed between a stationary source of radiation, such as a beam of gamma ray radiation or X-ray radiation, and a stationary detector. The radiation transmitted through the object is detected and measured. Radiographic images of the contents of the object may be generated based on the detected radiation after scanning. The images may show the shape, size, and varying densities of the contents. Contraband materials, including special nuclear materials, conventional explosives, and drugs, may thereby be potentially identified. The smuggling of contraband onto planes, as well as the smuggling of contraband across borders and by ship in large cargo containers, are serious threats.
Standard cargo containers are typically 20-50 feet long (6.1-15.2 meters), 8 feet high (2.4 meters), and 6-9 feet wide (1.8-2.7 meters). Air cargo containers, which are used to contain a plurality of pieces of luggage or other cargo to be stored in the body of an airplane, may range in size (length, height, width) from about 35×21×21 inches (0.89×0.53×0.53 meters) up to about 240×118×96 inches (6.1×3.0×2.4 meters). Large collections of objects, such as many pieces of luggage, may also be supported on a pallet. Pallets, which may have supporting side walls, may be of comparable sizes as cargo containers. As used herein, the term container is meant to include, but should not be limited to, standard cargo containers, air cargo containers, pallets, luggage, and handheld carry-ons.
Atomic bombs and “dirty bombs,” which may use a conventional trigger and conventional explosion to disperse radioactive material over a wide territory, are examples of nuclear devices that may be smuggled in cargo conveyances and smaller objects. Radioactive, fissionable, fissile, and fertile materials that may be used to manufacture nuclear devices, may also be similarly smuggled in such objects. Fissile materials, such as uranium-235, uranium-233, and plutonium-239, may undergo fission by the capture of a slow (thermal) neutron. Fissionable materials include fissile materials, and materials that may undergo fission by capture of fast neutrons, such as uranium-238. Fertile materials may be converted into fissile materials by the capture of a slow (thermal) neutron. Uranium-238, for example, may be converted into plutonium-239. Thorium-232, for example, may be converted into uranium-233. Fissionable, fissile, and fertile material may be referred to as “nuclear material.” These devices often include control and/or trigger electronics, such as timing devices or communications devices, that are used to detonate the explosive and/or trigger the nuclear device.
A variety of techniques are used to locate nuclear devices, nuclear materials, radioactive materials (that may not be nuclear materials), hazardous chemicals, and hazardous biological agents in containers. While manual inspection of the contents of the objects in a container may be effective in identifying hazardous targets, manual inspection is time consuming and costly. Identification of radioactive materials and nuclear devices and other weapons may be accomplished by passive inspection systems, such as a radiation detector.
Active systems that employ radiation to scan cargo and containers are also known. In one example of an X-ray scanning system, U.S. Pat. No. 5,524,133 discloses scanning systems for large objects, such as freight in a container or on a vehicle. In one embodiment, two stationary sources of X-ray radiation are provided, each emitting a collimated fan beam. The sources facing adjacent sides of the freight and the fan beams are perpendicular to each other. A stationary detector array is located opposite each source, on opposite sides of the freight, to receive radiation transmitted through the freight. The material content of the freight may be thereby analyzed. Additional radiation systems for inspecting large cargo are described in U.S. Pat. Nos. 6,292,533, 5,917,880, and 5,638,420, for example.
Likewise, in U.S. Pat. No. 6,347,132 B1, a high energy X-ray inspection system for detecting nuclear weapons materials is described wherein an object is scanned by a high energy X-ray fan beam or pencil beam. To obtain additional information about the contents of the luggage, radiation detectors may be provided to detect scattered radiation, as described in U.S. Pat. No. 5,642,394, for example. Systems may combine detection of scattered radiation with the detection of transmitted radiation.
Additionally, techniques such as computed tomography (“CT”) enable the reconstruction of the cross-sectional images of luggage contents, facilitating the identification of the items in the luggage, as discussed in U.S. Pat. No. 5,367,552 for example. CT images also provide higher resolution, greater image contrast and greater sensitivity to characteristics of the object being scanned, than radiographs. Scanning methods, such as CT, may be used to generate detailed images of identified objects for additional investigation.
In contrast to the cargo container size ranges, typical airport scanning systems for carry-on bags have tunnel entrances up to about 0.40×0.60 meters. Scanning systems for checked luggage have travel openings that are only slightly larger. Since only bags that fit through the tunnel may be inspected, such systems cannot be used to inspect cargo containers. The low energies used in typical X-ray luggage and bag scanners, which are typically in the keV range, are also too low to penetrate through the much larger cargo containers. In addition, many such systems are too slow to economically inspect larger objects, such as cargo containers.
The interaction of radiation with material also varies based on the varying effects of Compton scattering and pair production caused by the interaction of the radiation photons of differing energies with the different materials. A ratio of the attenuations of radiation beams detected at two different energy levels may therefore provide further information about the atomic numbers of the material through which the radiation beam passes. Dual energy systems, which scan an object with two radiation beams at different energy levels, may enable better detection of plastic materials and illegal drugs, for example, than single energy systems. U.S. Pat. No. 5,524,133, which is incorporated by reference herein, describes a dual energy technique for identifying contents of an object.